In elevators having a rated car speed exceeding 1 m/s, sliding safety gears are normally used as precautions when the elevator speed for some reason increases too much. The sliding safety gears grip the guide rails, of which there are usually two or four. In cases where each guide rail has its own sliding safety gear, the safety gears are mutually synchronized via separate synchronizing levers. The sliding safety gear is provided with a sliding surface which has a high friction coefficient and is pressed against the guide rail when the safety gear is activated, thus decelerating or stopping the elevator car by means of friction.
Various elevator safety gear structures have been developed. On of the most common is a large U-shaped spring made of spring steel, in which a wedge is thrust into the gap between the spring ends as it grips the guide rail. In addition, many safety gears have a separate release wedge by means of which the wedge is released from the guide rail after the safety gear action. The releasing is effected by raising the elevator car.
An example of the state of the art is also Finnish patent no. 74686, corresponding to German patent DE 3715098 and American U.S. Pat. No. 4,819,765. To stop the elevator car unit, both the car unit and the counterweight can be provided with safety gears e.g. as presented in patent 74686 and, to ensure safe operation in door zones, the overspeed governor can be provided with an electrically operated triggering device for switch-over to low speed. However, this is an expensive solution and takes up plenty of room because a safety gear is needed for the counterweight as well. In a sliding elevator safety gear according to this patent, standard parts are used and the wedge chamber is provided with a power means which imparts to the counter wedge a force acting substantially in the direction of the guide surface. The distance between the upper edges of the guide surfaces is equal to or greater than the distance between the lower edges of the corresponding guide surfaces. The force of the power means is generated by a spring. This patent does not accomplish compensation of the changes of friction on both sides but only on the side of the spring. Moreover, the clearances are relatively small.
In certain countries, the elevator regulations have been revised to prevent the occurrence of the following accidents:
An elevator car hits the ceiling of the elevator shaft after running up at an overspeed.
A passenger is crushed by the doorway structures of an elevator that has left a floor with doors open.
The new regulations also allow more freedom for the design of the safety equipment as they permit the use of non-mechanical solutions as well.